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EXPERIMENTAL GENERAL SCIENCE 



128. Lenses. Circular pieces of glass known as lenses, by 

 deflecting the light rays either toward or away from one 

 another, make objects appear to be larger or smaller than they 

 really are. There are several forms of lenses, but the two 

 most frequently used are the double convex lens, which is thick- 

 est in the middle and curves uniformly to the edge, and the 



double concave lens, which is just 

 the reverse of this. When rays 

 of light pass through a double 

 convex lens, owing to the curva- 

 ture of its surfaces, they are 

 directed toward the thicker part 



^ ^ ne ^ ens anc ^ m ade to fall on a 

 single spot, or, as we say, are 



FIG. 53. How curvature of 

 the lens affects direction of the 

 light rays. (Tower, Smith and 

 Turlon.) 



FIG. 54. Bending of a light ray by 

 atmosphere; sun appears to be higher 

 than it really is. 



brought to a focus. The very bright spot of light that 

 appears when such a lens is held a short distance above any 

 convenient surface is an illustration of this fact. That heat 

 rays as well as light rays may be focused is well known. The 

 double convex lens is sometimes known as a burning glass be- 

 cause the heat rays falling on it may set fire to paper when 

 brought to a focus. When light from an object passes through 



